>>>"A DC9 is a DC9-80 is an MD80 is an MD 95 is a 717 >>>- just like the 747-400 still is a 747 and both 737->>>100 and -900 belong to the same 737-family"
Some of what you said is true....
The type rating is good for the MD-80/90/717/DC-9.
However, the majority of the DC-9s were built in the sixties and early seventies. We have two of the oldest active DC-910 series on our line (N700ME/N500ME). As you might well imagine, the cockpit configuration is quite different between the DC-9s and MD-80 series. Our DC-9s tend to be analogue, and they are FMS--free. The MD-80s, on the other hand, have an EFIS. The 717 is an entire glass cockpit. There is a seperate training program for our MD-80 crews even though they hold a DC-9 type rating. Vice versa for the DC-9 guys that upgrade to the MD-80. In our company you typically bid a line for one or the other.
Here are some more differences (basically off the top of my head):
(1). "Typical" crusing speed for the 10 series (561kts)
(2). The 10 series are not equipped with leading edge slats.
(3). The cruising speed for the 20 series is 557 kts because of the longer wingspan (from the 30 series) on the 10 series body. You have a trade off between take-off and landing performance and cruise as the longer wing span means increased drag.
(4). The 30 series is the most prolific of the DC-9s and utilizes the JT8D-7 or JT8D-9 which the larger of the two adds about 500lbs of thrust. Note that all series 50s and below with the D9s are required to be hush kitted.
(5). The MD-81 has the JT8D-209 and take off weight of 140k.
(6). MD-82 has the JT8D-217s and range of 1875nm.
(7). MD-88 has a T/O weight of 149.5k. It is powered by the D-217C and has a range of 2235nm. It used to be that the MD-88 had the "signature" screw driver tail cone, but someone (maybe AAR90) mentioned that other MD80 series were adding that feature. The MD-88 has a spacious "walk in" galley.
(8). The MD-83 has the longest range of the MD80 family at 2345nm and a T/O weight of 160k due to higher fuel capacity.
(9). The 717 has a 1430 nm range and is powered by RR BR715--all glass cockpit too.
There's some other differences (I didn't really get into the MD-87/MD-90). But, by and large, the engines are tailmounted, the seating is 2*3 --unless you fly us or Legend
. And yes, the type rating is good for the 9 and the MD-80. Look closely, however, and there is a very vast difference.